This invention relates to a process of producing a high-methane gas, which can be substituted for natural gas, from a primary gas which has been produced by a gasification of coal, tar, or heavy residual oil under superatmospheric pressure and which has been purified to remove catalyst poison and has been scrubbed to remove carbon dioxide to a residual content below 2% by volume. Heavy residual oils are hydrocarbons which boil above 250.degree. C.
Such gas which can be substituted for or mixed with natural gas should contain at least 90% by volume methane and less than 2% by volume hydrogen. The gas should be virtually free of carbon monoxide. A residual carbon dioxide content is generally not disturbing. It is known that coal, tar, and heavy residual oil can be degasified with water vapor and oxygen under superatmospheric pressure and at elevated temperatures to produce a raw gas which contains carbon oxides in excess of its hydrogen content. The raw gases produced by the gasification of coal with water vapor and oxygen under a pressure of 20-80 kg/cm.sup.2 absolute pressure generally have a high CO.sub.2 content (28-32% by volume) and CO content (15-20% by volume) and a relatively low H.sub.2 content (35-44% by volume). The gasification of tars and residual oils is carried out at higher temperatures (about 1100.degree.-1500.degree. C) than the gasification of coal and results in a raw gas which contains 3-6% by volume CO.sub.2, 46-50% by volume CO, and 40-48% by volume H.sub.2.
These oxides of carbon contained in the gas produced by gasification are hydrogenated to produce methane preferably in contact with a high-activity hydrogenation catalyst, which contains metallic nickel as its active component. The reaction of CO and CO with hydrogen to produce methane and water is also referred to as methanation and takes place in accordance with the following reaction equations: EQU CO + 3 H.sub.2 .revreaction. CH.sub.4 + H.sub.2 O EQU co.sub.2 + 4 h.sub.2 .revreaction. ch.sub.4 + 2 h.sub.2 o
and is accompanied by a heat change of 49.3 kcal/mole CO and 39.4 kcal/mole CO.sub.2.
It is apparent that 3 moles H.sub.2 per mole CO are required to hydrogenate carbon monoxide to form methane and 4 moles H.sub.2 are required to hydrogenate 1 mole CO.sub.2. For this reason the volume ratio EQU H.sub.2 : (3 CO + 4 CO.sub.2)
which is defined as the stoichiometric ratio, must be equal to or larger than 1 for a complete conversion of the carbon oxides to methane. Because CO.sub.2 can be scrubbed in a simple manner from the product gas after the synthesis of methane, CO.sub.2 need not be hydrogenated to methane as completely as possible. On the other hand, a complete hydrogenation of CO is essential because CO which has not been hydrogenated and remains in the product gas is undesired and can be removed only with great difficulty. To increase the above-mentioned stoichiometric ratio H.sub.2 : (3 CO + 4 CO.sub.2), part of the CO.sub.2 or in most cases all of the CO.sub.2 must be removed. This is accomplished by the purification of the gas which is required for a complete removal of the catalyst poisons (H.sub.2 S, organic sulfur compounds, HCN, and NH.sub.3) from the gas. The gas is scrubbed with a physically or chemically acting absorbent to remove the carbon dioxide to a residual content of a few percent.
Even when the CO.sub.2 has been scrubbed out, the H.sub.2 : 3 CO volume ratio is still below the required minimum of 1. To increase the ratio to that value, part of the carbon monoxide is subjected to the known catalytic shift conversion reaction with water vapor according to the reaction equation EQU CO + H.sub.2 O .fwdarw. CO.sub.2 + H.sub.2
to produce hydrogen and CO.sub.2.
This results in the previously known process sequence, which is shown in FIG. 2 of the drawing described below.
This process has the significant disadvantage that steam at a high rate must be supplied for the shift conversion and only 20-30% of this steam directly participate in the shift conversion reaction. The remaining 70-80% of the steam are merely ballast, which serves mainly to limit the adiabatic temperature rise. The extraneous generation of this ballast steam and its subsequent removal by condensation involve considerable costs.